Neutrophils are phagocytic white blood cells, and a deficiency of their numbers ("neutropenia") predisposes to bacterial and fungal infection. The two principal inherited forms of human neutropenic bone marrow failure are cyclic neutropenia (CN) and leukemia-predisposing, severe congenital neutropenia (SCN). "Benign ethnic neutropenia" is common to individuals of African descent and may also have health consequences. Mutations of the gene ELA2, encoding neutrophil elastase (NE), cause CN and are the most frequent cause of SCN. Recent findings that canine CN is caused by mutations of the AP3 transporter and that some SCN may result from ELA2 promoter variants or mutations of the transcriptional repressor Gfi1, both of which lead to over-expression of NE, suggest a hypothesis for neutropenia in these and other bone marrow failure syndromes: NE is a transmembrane "cargo" protein for AP3. Mutation of NE's transmembrane domains causes too much NE to accumulate in granules, resulting in CN. Mutation of NE's AP3-recognition signal, or of AP3 itself, mislocalizes NE to the plasma membrane. Similarly, over-expression of NE consequent to ELA2 promoter variants or Gfi1 mutations overwhelms normal AP3-mediated trafficking pathways and diverts NE to the plasma membrane. Three Specific Aims are directed toward testing this hypothesis, determining if a common ELA2 promoter variant in the African-American population associates with benign ethnic neutropenia, and identifying additional neutropenia genes: 1 Characterize NE's membrane relationship: 1.1 Develop antibodies to predicted cytoplasmic and luminal surfaces of NE; 1.2 Test membrane-bound NE for altered catalysis on model and biological substrates; 1.3 Examine the evolutionary conservation of AP3 interactions in other species and for other proteases. 2 Evaluate mistrafficking of NE as a general mechanism for neutropenia: 2.1 Identify potential substrates and cofactors of mislocalized NE; 2.2 Inspect NE trafficking in other neutropenic disorders. 3 Discover other mutations that may cause neutropenia through over-expression of NE: 3.1 Determine if ELA2 promoter variation contributes to neutropenia; 3.2 Measure the frequency of the ELA2 C-199A allele in individuals of African descent with benign ethnic neutropenia; 3.3 Identify new neutropenia genes.